Condition before being touched

My father-in-law gave my boy a 1999 350 Big Bear. It's been sitting in a barn for a bunch of years. It runs great! My boy has been riding a couple weeks now and it has developed a squeal coming from the front end when you get off the gas at speed. No noise during acceleration. At first I thought brakes. I took it for a ride and I don't think it is the brakes. I feel it is the front prop shaft U-joints, front differential or one/both axles. I pulled the fill plug on the differential and I can't find anywhere how much gear oil should be in there. My gut says up to the fill opening. It doesn't have a manual. It's wet in there, just don't see any gear oil.

#1 Should I fill the differential with gear oil up to the fill opening?

#2 How do I check or know if the u-joints are bad? That looks like a "Big Bear" of a job if they don't need to be replaced.

#3 What else could it be? I don't think it is the front bearings. I jacked the front wheels off ground and no play in the right wheel. Very little play in the left wheel, looks like the lower control bushing is where that play is coming from. The rubber CV boots look original but all still intact. Wheels spin easily without any noise.

Thanks for any help!

I have an older (1987) Yamaha Big Bear 350.  I have arthritis in my thumbs.  Is it possible to replace the accelerator with a motorcycle accelerator (ie. twisting the handgrip)?  This would be more comfortable for me due to the arthritis.

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Clymer ATV Manual - Yamaha Raptor 700R
 

I bought my big bear in 2009, never had a bit of problem with it until last year. It quit starting but still would click when I tried to start it. I replaced the solenoid, the cdi module ( it was burned on one corner) and this year replaced the battery. I have all lights but still will not start and I do not have the clicking when I try to start it. Can anyone give me any ideas what to try next? I haven't had any luck finding videos about the 2008 Big Bear. I'm trying to avoid the expense of sending it to the shop. 

Damn guys its been a minute since posting here! Hows everyone doing with covid-19 floating around? I hope everyone is staying safe and washing your hands! 
 
Its been a minute! Myself and Jon got requested to do this video only 1000 times! Eventually Finntrail started a new Blog and got some of their sponsored ATV riders make a video for them so here is ours! We got lots of parts on our Yamaha Grizzly 700 everything you see on my Yamaha Grizzly 700 you will also see on Jons Yamaha Grizzly 700. Thanks for watching and make sure you Like, Comment and Subscribe!
 
P.s I also said the 1992 Big Bear had a belt lol